In general, a complementary metal oxide semiconductor (CMOS) image sensor is a semiconductor device that converts an optical image to an electrical signal. The image sensor is basically classified into a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor.
Among the image sensors, the CCD image sensor is the semiconductor device that each of metal-oxide-silicon (MOS) capacitors is placed in close proximity and charge carriers are stored in and transferred. The CMOS image sensor adopts a switching method for sequentially detecting outputs of many metal oxide semiconductor (MOS) transistors constituent with the number of pixels based on CMOS technology.
The CMOS image sensor is cheaper than the CCD image sensor and has power consumption as much as 1/10 of that of the CCD image sensor.
A conventional CMOS image sensor which process image data (analog signals) acquired from pixels is described in FIG. 1.
FIG. 1 is a block diagram showing a conventional COMS image sensor. In detail, FIG. 1 describes a data path transmitting an analog data generated a pixel array in the conventional COMS image sensor.
As shown in FIG. 1, the conventional CMOS image sensor includes a pixel array 10, a correlated double sampling (CDS) block 20 and an analog signal processor (ASP) 30. The pixel array includes plural red (R), green (G) and blue (B) pixels arranged in an M×N matrix. The correlated double sampling (CDS) block 20 including CDS circuits at each column is located at a lower side of the pixel array 10. The analog signal processor (ASP) 30 processing the analog signals outputted from the CDS block 20 is located at a right side of the pixel array 10.
The output signals of the CDS block 20 are transferred to the ASP 30 through an analog data bus. The analog data bus is constituent with a first analog data bus 52 and a second analog data bus 54.
The output signals of the CDS block 20 are loaded on the first analog data bus 52 or the second analog data bus 54 by a selecting block 60 which is controlled by a select signal, e.g., CS0, generated from a column driver 40. The selecting block 60 includes plural switches for selectively delivery the outputs of CDS block 20 into one of the first analog data bus 52 or the second analog data bus 54.
The ASP 30 has an ASP-A 32 and an ASP-B 34 to amplify each analog data transferred through the first analog data bus 52 and the second analog data bus 54.
The CDS block 20 samples a reset signal and a data signal from each pixel and supplies the sampled reset and data signals on the analog data bus. Then, the ASP 30 calculates a difference between the reset signal and the data signal and amplifies the difference. Accordingly, an analog pixel data of a captured object can be obtained.
Further, the column driver 40 receives a column address to thereby generate the select signals, e.g., CS0.
Hereinafter, the process of the CMOS image sensor is described as follows.
When the CMOS image sensor reads pixel data, the pixels arranged along one row of the pixel array 10 are transferred to the CDS circuits of the CDS block 20 at once and at the same time (at the same clock). Under the control of a column driver 40, output signals of the CDS circuits are loaded on one of the first analog data bus 52 and the second analog data bus 54 by a selecting block 60 and are sequentially transferred to the ASP 30.
For example, a sequence that pixel data generated from a first row and a second row in the pixel array 10 are loaded on the first analog data bus 52 and the second analog data bus 54 is as the following Table 1.
TABLE 1The first rowThe second rowThe firstB11B13B15. . .G21G23G25. . .analog data busThe secondG12G14G16. . .R22R24R26. . .analog data bus
Referring to Table 1, among pixel data located at the same row, pixel data corresponding to every odd column line are loaded on the first analog data bus and pixel data corresponding to every even column line are loaded on the second analog data bus.
In detail, even the same green (G) pixel data pass through the path A on the first analog data bus or the path B on the second analog data bus according to the located column line. Also, the Red (R) and Blue (B) pixel data pass through the path A or path B according to the located column. That is, pixel data are transmitted through different paths according to pixel location, not a type of pixel. Thus, even the same type pixel data have an offset because of passing through the different paths.
As above described, if the offset occurs because of a path difference, an offset noise appears in a real image to thereby deteriorate an image quality.